Nutrient flow between land and sea:

bears and salmon

Background

• Salmon runs have been declining for decades

• Many runs are endangered or completely extirpated

• Can have economic/cultural/ecological repercussions

Background • Salmon runs can yield as much as 5.4 x107 kg of

biomass kg of biomass

• A significant pulse of P,N,Ca and other macroelements

• Distribute nutrients as far as 1000km away with a 2000m gain in elevation (Columbia River to Redfish Lake, Idaho)

How do we know?

• Radioisotope data shows the nutrient movement

• One of few examples of documented nutrient flow between marine, freshwater, and terrestrial ecosystems.

Salmon

Female salmon prepare a redd in the streambed and lay eggs

Males fertilize the eggs and they hatch in 8-12 weeks

The small fry live in their natal stream for 1-3 years, eating aquatic insects

• They grow from fry to parr

• At about 3 years they go to the open ocean

• They live in the open ocean for 1-7 years as adults, eating fish and growing larger.

• Then they return to the exact same place they were born

to spawn.

Salmon

Returning to their natal grounds is

challenging! • Dams and fish ladders• Waterfalls and rugged rivers • Miles upstream against the current• Predators await them• Not eating during the entire journey

Bear feeding

Bears are in hyperphagy- they need weight for winter torpor period

They will need to put on hundreds of pounds to support hibernation through the winter.

Bear feeding

Bears are under pressure to consume as many calories as possible in a short time

Therefore, they eat the most fat and protein rich portion of the fish; the brains and roe

Then throw the fish onto the shore,

distributing marine nutrients to a

terrestrial system.

Bear feeding

• K of bears increases with salmon availability• 80x denser in coastal areas than interior • Bears distribute 83-84% of salmon onto shores • 96% of nitrogen excreted in urine, 3% by feces (spatial

effect)

Different endings

Some cannot complete journey and die in transit

Some make it, spawn, and die in the water column, adding nutrients that support algae and stream insect populations

Insects

• The fish become a source of protein for the developing insects

• Flies oviposit in the carcasses (50% infection rate)

• 50,000 maggots per carcass, can consume an entire fish in 5 days

Insects

• Blowflies dominate salmon carrion and consume about 90% of the 4000 kg of carcasses abandoned by bears

• With a single carcass, insect abundance increases several levels of magnitude

• Macroinvertebrates derive up to

75% of their nitrogen from salmon

Insects

• Young salmon who ate aquatic insects that were higher in density were larger due to salmon nutrients

• Young salmon had a higher proportion of lipids allocated to energy reserves

• This may lead to higher survival rates and fecundity

Other carnivores in the system

• Eagles and otter eat the salmon along river banks. • Greater food availability allows for higher fecundity and

more offspring in following seasons

Other animal species

Bird community densities increase with the pulse of invertebrates due to salmon presence

Muscles of small mammals show marine nitrogen signatures through indirect assimilation via terrestrial vegetation

Humans in the system

Anglers

Tribal fisheries

Commerical fisheries

Hatcheries

Vegetation

Plants on the shore get a sudden influx of nitrogen, a limiting nutrient.

This allows for growth of willow, sedges, and trees along the river bank.

These act to shade the river and provide shady pools for fry to grow away from predators

Tree growth is measured in rings that reflect accelerated growth that coincide with years of particularly high nutrient availability (large salmon runs).

Vegetation

Salmon affect the quality of in-stream habitat through:

• Shading • Sediment filtration • Nutrient contribution • Production of woody debris • Improved spawning and rearing habitat for

subsequent salmon generations

Plant response

• The salmon nitrogen is incorporated into streamside plant life

• 500 m from streams vegetation showed marine nitrogen radioisotopic signatures

• Trees and shrubs near spawning streams derive 22-24% of their foliar nitrogen from spawning salmon

• growth rates were significantly higher near spawning streams- more habitat

Plant response

• Salmon derived wood samples

from old-growth riparian areas

show marine nitrogen

• Show that this nutrient transfer

has been happening for centuries

Back to the start

The fish that make it through all those challenges, and back to their natal grounds

Create a redd and lay their eggs, and males fertilize the eggs

The males and females then die near the redd, and their nutrients supply the local aquatic invertebrates and algae with nitrogen for growth

Integrated approach to management

• Climate cycles • Salmon ecology • Riparian vegetation • Flooding and hydrologic cycles• Predator ecology and nutrition • Marine nutrient flowpaths • Feedback loops • Economic drivers